Telephone wires connect homes and businesses to service providers and telephone companies. In many cases, these wires are able to support data rates between the user and service provider which allows for services such as internet access, video, and digital telephony.
Asymmetrical digital subscriber line (ADSL) is a common digital subscriber line (DSL) specification which allows for the transport of data over telephone wires. ADSL uses discrete multitone (DMT) modulation, a form of multicarrier modulation, to effectively divide a channel into a number of parallel subchannels, each of which can be optimized to maximize the overall data rate.
DMT allows for simple frequency-domain equalization if the memory of the channel does not extend past the length of the prefix. The prefix of a DMT symbol is formed by appending the last P samples of the fast Fourier transform (FFT) of the data to the beginning. However, for many practical systems (ADSL included), the channel memory is longer than the prefix. To address this, a time-domain equalizer (TEQ), usually a filter, is typically used to effectively shorten the overall channel (channel plus TEQ) such that the overall channel memory is less than or equal to the prefix length.
Because of the wide variety of channels and noise environments encountered in ADSL deployments, it is difficult to design a single TEQ structure which appropriately shortens all channels and allows for near capacity data rates with reasonable complexity.